In recent years, in accordance with increases in the memory capacity of magnetic disk drives, track density has been increasing. The increase in track density narrows width of tracks (i.e., the track pitch) on a disk (magnetic disk).
Where the track pitch is narrowed, data already written to tracks adjacent to a target track may be magnetically degraded in accordance with writing data to the target track with a head (magnetic head). This is caused by a component, called leakage magnetic field, which is included in a magnetic field generated by the head and makes no contribution to data written to the target track.
The influence of the leakage magnetic field on a certain track (hereinafter referred to as “track T”) is accumulated whenever a data write is performed on tracks adjacent to track T. Accordingly, as the number of data writes to the adjacent tracks increases, the magnetization of track T decreases (i.e., demagnetization occurs). This phenomenon is called adjacent track interference (ATI).
As the demagnetization of track T advances due to ATI, the error rate in reading data from track T increases. Before long, even if an attempt is made to recover the data in track T by making full use of an error correction code (ECC), the data recovery will be difficult. As a result, the data will be lost. The number of data writes to adjacent tracks before the data in track T is lost varies in accordance with the write access pattern.
In recent magnetic disk drives, rewriting to restore the magnetization of track T (track refreshing) before track T becomes unreadable as a result of ATI has become essential. Such track refreshing restores the magnetization of track T.